netdev - Re: [PATCH bpf-next v10 06/10] bpf,landlock: Add a new map type: inode

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Date:   Wed, 31 Jul 2019 21:11:10 +0200
From:   Mickaël Salaün <mickael.salaun@....gouv.fr>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Mickaël Salaün <mic@...ikod.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Drysdale <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <jmorris@...ei.org>, Jann Horn <jann@...jh.net>,
        John Johansen <john.johansen@...onical.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Paul Moore <paul@...l-moore.com>,
        Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>,
        Stephen Smalley <sds@...ho.nsa.gov>, Tejun Heo <tj@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Thomas Graf <tgraf@...g.ch>, Tycho Andersen <tycho@...ho.ws>,
        Will Drewry <wad@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v10 06/10] bpf,landlock: Add a new map type:
 inode

On 31/07/2019 20:58, Alexei Starovoitov wrote:
> On Wed, Jul 31, 2019 at 11:46 AM Mickaël Salaün
> <mickael.salaun@....gouv.fr> wrote:
>>>> +    for (i = 0; i < htab->n_buckets; i++) {
>>>> +            head = select_bucket(htab, i);
>>>> +            hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
>>>> +                    landlock_inode_remove_map(*((struct inode **)l->key), map);
>>>> +            }
>>>> +    }
>>>> +    htab_map_free(map);
>>>> +}
>>>
>>> user space can delete the map.
>>> that will trigger inode_htab_map_free() which will call
>>> landlock_inode_remove_map().
>>> which will simply itereate the list and delete from the list.
>>
>> landlock_inode_remove_map() removes the reference to the map (being
>> freed) from the inode (with an RCU lock).
>
> I'm going to ignore everything else for now and focus only on this bit,
> since it's fundamental issue to address before this discussion can
> go any further.
> rcu_lock is not a spin_lock. I'm pretty sure you know this.
> But you're arguing that it's somehow protecting from the race
> I mentioned above?
>

I was just clarifying your comment to avoid misunderstanding about what
is being removed.

As said in the full response, there is currently a race but, if I add a
bpf_map_inc() call when the map is referenced by inode->security, then I
don't see how a race could occur because such added map could only be
freed in a security_inode_free() (as long as it retains a reference to
this inode).

--
Mickaël Salaün
ANSSI/SDE/ST/LAM

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